This invention relates to a solid bowl centrifuge for separating solids from a liquid, for different applications, such as in a waste water treatment facility. In particular, the invention relates to a weir and choke plate assembly for a solid bowl centrifuge.
Conventional solid bowl centrifuges typically are embodied as helical conveyor centrifuges having a screw within a bowl. The screw and bowl are coaxial and rotate independently of each other. The screw rotates to transport solids in a liquid solids mixture to a solids discharge end of the centrifuge. The rotations of the screw conveyor and the bowl apply a centrifugal force causing the liquid and solids in the bowl to form an annular ring having an outer surface against the wall of the bowl and an inner cylindrical ring surface (the “pond level”) that surrounds a gas filled void at the center of the centrifuge.
Solids in the rotating bowl tend to settle radially outward on the bowl wall and are moved by the screw to the solids discharge end of the centrifuge. The clarified liquid, also referred to as centrate, is radially inward and flows to a liquid discharge at a head wall which is at an end of the centrifuge opposite to the solids discharge end.
Weir assemblies are attached to the head wall of the centrifuge. The weir assembly typically includes a passage for the discharge of liquid centrate from the bowl of the centrifuge. The conventional weir assembly is adjustable to regulate the level of the liquid in the bowl. For example, German Patent Publication DE 1 183 023 discloses a weir assembly having two concentric ring-disks forming a V-shaped drain opening that is radially adjusted during operation of the centrifuge to regulate the liquid level in the centrifuge bowl. Other adjustable weir assemblies and weir aperture geometries are disclosed in DE 1 452 260. Similarly, DE 39 21 327 discloses weir elements for weir assemblies formed of flaps, slides and wedges arranged adjacent drain openings in the head wall of a centrifuge. These weir elements rotate with the bowl of the centrifuge and are adjusted radially by a set collar. The pond level of the liquid centrate in the centrifuge bowl, as well as the quantity of the centrate being drained, is regulated by adjusting the radial position of the set collar.
DE 102004019368 discloses a solid bowl centrifuge with an adjustable weir system having adjacent weir plates in which the inner weir plate rotates with bowl and the outer weir plate is fixed. The outer weir plate does move in a small set-wise rotational movement that allows for an adjustment of the effective gap between the outer and inner weir plates. The adjusting mechanism for the outer weir plate is eccentrically mounted with respect to the rotating centrifuge. The centrate flows through the gap between weir plates.
DE 43 20 265 discloses a solid bowl centrifuge with an adjustable weir having a non-rotating choke plate, also referred to as a throttle plate. The choke plate is axially displaced, arranged outside of the bowl, and is adjacent a rotating liquid drain openings in the bowl. The choke plate is in a plane parallel to the drainage cross sections for the liquid and of the liquid pond level in the bowl of the centrifuge. As the gap between the choke plate and drain openings decreases, the liquid drainage flow resistance increases and the liquid pond level in the centrifuge increases by extending radially inward during centrifuge operation.
DE 102 03 652.7 discloses a weir discharge with a rotating weir plate and a non-rotating choke plate that creates a liquid centrate discharge opening in which at least one nozzle is assigned to an outlet for discharging clarified liquid from the drum. Energy may be saved depending on the relative angle of the nozzles. Another energy savings concept for a weir is disclosed by WO2004035221.
U.S. Patent Application Publication US2004/0058796 discloses a weir discharge system where the centrate is directed outwards through a non-rotating annular cup with at least one opening that is connected to the centrifuge housing. The position of the annular cup can be adjusted during operation and thereby the centrate flow and liquid pond level are controlled.
DE 37 28 901 C1 discloses a centrate discharge system having inner and outer rotating weir plates between which is a fixed gap opening. The inner weir plate has a larger inner diameter than does the outer weir plate. The fixed gap between the weir plates creates a flow path for the centrate. After the gap is closed by a flange, the liquid level rises until the centrate flows over the edge of the outer weir.
U.S. Pat. No. 5,169,377 discloses a weir discharge system in which the liquid outflow is regulated by an annular discharge gap between rotating ring-weir plates. The outer ring weir plate moves axially to change the size of the gap between the ring and a circular discharge opening.
A weir and choke plate assembly should provide one or more of an easy adjustment of the liquid pond level in the centrifuge, a relatively low torsion moment to drive the assembly, a gas seal to isolate the gas filled void in the centrifuge from ambient atmosphere, and a decanting function in which the liquid level in the centrifuge is periodically raised to a radially inward drain opening.